In general, butterfly valves are arranged to be normally biased towards a closed position by, inter alia, two restoring elements which are independent of one another. In the case of mechanical systems operated by, for example, a Bowden cable linkage, the restoring elements comprise two separate springs. In a conventional electronically controlled butterfly valve, the two restoring elements comprise one spring and an electronic adjusting element.
If one of the two restoring elements should fail in operation, the second restoring element (provided as a duplicate) always fulfills the safety critical closing function--it being essential that, in the event that the driver is not depressing the accelerator pedal, the engine is always returned to a minimum speed (idling) condition. Once one restoring element has failed, there is, of course, no duplication available, and it is essential for the driver to be warned and for him to seek a workshop for the faulty part to be replaced. This is especially important in the case of electronic systems, since a fault in the return spring in the adjusting element does not readily become apparent immediately because the pedal forces remain unaltered as a result of the mechanical decoupling between the pedal and the butterfly valve unit.
The Abstract of JP-A-60-79130 shows a throttle flap control device. The current of the driving motor, which controls the position of the throttle flap according to the output of an accelerator position sensor and a throttle position sensor, is detected as a reference value. The detected current is not used for checking purposes.
From the document "VDI-Berichte" No. 687, 1988, pages 365 to 385, it is known that for self-checking purposes, the current which is supplied to the actuators of an engine is monitored. A failure of the system is recognized when the current exceeds an upper or a lower limit value.
However, the above-cited problems concerning mechanical fault conditions in electronic systems cannot be avoided. The mechanical fault conditions may not become apparent by comparing the detected current value with the upper or the lower limit value as described in the above-cited document, because the electronic system controls the throttle flap position according to the accelerator position even for example in the absence of a return spring. The detected current value may not exceed the predetermined upper or lower nominal limit value.